Triggered Star Formation in a Turbulent ISM Proceedings IAU Symposium No. 237, 2006 c 2007 International Astronomical Union B. G. Elmegreen & J. Palouˇs, eds. doi:10.1017/S1743921307002050 Young stellar clusters triggered by a density wave in NGC 2997 P. Grosbøl1, H. Dottori2 and R. Gredel3 1European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, DE email: [email protected] 2Instituto de F´ısica, Univ. Federal do Rio Grande do Sul, Av. Bento Gon¸calves 9500, 91501-970 Porto Alegre, RS, BR email: [email protected] 3Max-Planck Institut f¨ur Astronomie, K¨onigstuhl 17, 69117 Heidelberg, DE email: [email protected] Abstract. Bright knots along the arms of grand-design spiral galaxies are frequently seen on near-infrared K-band images. To investigate their nature, low resolution K-band spectra of a string of knots in the southern arm of the grand design, spiral galaxy NGC 2997 were obtained with ISAAC/VLT. Most of the knots show strong Brγ emission while some have H2 and HeI emission. A few knots show indications of CO absorption. Their spectra and absolute K magni- tudes exceeding -12 mag suggest them to be very compact, young stellar clusters with masses up 4 to 5 × 10 M. The knots’ azimuthal distance from the K-band spiral correlates well with their Brγ strength, indicating that they are located inside the co-rotation of the density wave, which triggered them through a large-scale, star-forming front. These relative azimuthal distances sug- gest an age spread of more than 1.6 Myr, which is incompatible with standard models for an instantaneous star burst. This indicates a more complex star-formation history, such as several bursts or continuous formation. Keywords. galaxies: spiral, galaxies: star clusters, infrared: galaxies, techniques: spectroscopic 1. Conclusions Several K-band emitting knots, aligned along the southern arm of NGC 2997, were observed by Grosbøl et al. (2006). The analysis of the low-resolution, near-infrared spectra gave the following main results: • their absolute K magnitudes exceed -12 mag, • K-band emission spectra are similar to cocoon-enshrouded star forming regions, 4 • age range is 7-10 Myr and masses are up to 5 × 10 M, • sizes and masses are similar to that of W49A in our Galaxy, • alignment with spiral pattern along almost 4 kpc is striking, and • locations of the knots are consistent with being inside co-rotation of spiral pattern. This suggest a well-synchronized mechanism of piling up the material of which they were formed and for the triggering of star formation similar to the scenario of density wave propagation. Agreement between large-scale, synchronously triggered star formation and knot age differences requires several bursts or continuum star formation instead of a single burst within the knots. Reference Grosbøl, P., Dottori, H. & Gredel, R. 2006, A&A 453, L25 417 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.8, on 28 Sep 2021 at 10:09:09, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921307002050.